1. Field of the Invention
The present invention generally relates to navigation apparatus and, more particularly, is directed to a navigation apparatus which makes effective use of a global positioning system (GPS) to detect an azimuth angle, a position, a velocity and so on of navigation vehicles, such as ships, automobiles or the like.
2. Description of the Prior Art
As is conventional, a ship or the like is provided with a gyro compass and a magnetic compass as an apparatus for measuring its azimuth so that, under any conditions, she can sail safely while measuring its own azimuth constantly.
However, the gyro compass has the disadvantage such that it needs an actuation time as long as one hour or more. Also, the magnetic compass points to the north of terrestrial magnetism so that the ship's azimuth pointed by the magnetic compass is unavoidably deviated from the true north.
Recently, a global positioning system (hereinafter simply referred to as a GPS) navigation system is proposed to obviate the aforesaid disadvantages and shortcomings of the prior art and which can constantly detect the position of a navigation vehicle such as a ship or the like. The GPS system can measure the position of the navigation vehicle in a three-dimensional fashion on the basis of data supplied thereto from three GPS satellites or more. It is expected that this GPS system will be able to be employed by using a commercially available code, a so-called C/A code in the 1990s until which the launch of the GPS satellite is finished.
In the GPS signal processing based on the above ordinary measuring process, only the position of the navigation vehicle can be measured and a large error occurs in the position measuring process. As a consequence, the azimuth of the navigation vehicle cannot be measured according to the GPS. On the other hand, a method of calculating the azimuth angle of navigation vehicle is presented. According to this method, the azimuth angle of navigation vehicle is calculated by a two-position difference high accuracy simultaneous measuring method which measures a phase difference of GPS satellite radio waves used in the measuring called a differential GPS system.
A principle of this measuring method will be described below with reference to FIG. 1.
In FIG. 1, reference numerals 1 and 2 depict reception antennas installed on a navigation vehicle (not shown) such as a ship, an automobile, an airplane or the like, for example. A base line length, i.e., a distance L between the two antennas 1 and 2 is known. Radio waves from these antennas 1 and 2 are supplied to a GPS azimuth computing unit 3 which calculates an azimuth angle component .phi. of the navigation vehicle on the basis of the following processing.
As shown in FIG. 1, let it be assumed that a radio wave from a single GPS satellite 5 is simultaneously received by the antennas 1 and 2, At that time, due to the distance L between the antennas 1, 2 and the position of the GPS satellite 5, a distance difference shown by reference letter D in FIG. 1 is provided between the radio wave received at the antenna 1 and the radio wave received at the antenna 2. If a particular radio wave of a carrier is noticed, then this distance difference D can be measured as the phase difference (time lag). Accordingly, the distance difference D can be obtained by multiplying the phase difference with a wavelength of the radio wave. If the distance difference D is obtained, then the distance L is already known so that the azimuth angle .phi. of the navigation vehicle relative to the GPS satellite 5 can be calculated as: EQU .phi.=COS.sup.-1 (D/L) (1)
In this measuring process, a reception code is not always decoded.
An azimuth angle formed by a line connecting the GPS satellite 5 and the antennas 1, 2 and the true north (N) is calculated as follows:
After the radio wave from the GPS satellite 5 is received at the antenna 1, radio waves from at least other two GPS satellites or more (not shown) are received. Then, the C/A codes of the received radio waves are decoded and a transmission time and a reception time of the radio wave from the GPS satellite are calculated to thereby obtain a propagation time of radio wave from the GPS satellite. Then, a distance from the antenna 1 to the GPS satellite, accordingly, the distance from the GPS satellite to the navigation vehicle is calculated by multiplying the calculated propagation time with a wavelength of the radio wave. Since the equidistant position from one GPS satellite exists on the spherical surface whose radius is equal to that distance, three spherical surfaces from the three GPS satellites are calculated and an intersection point of the three spherical surfaces is calculated, thereby the position of the reception antenna 1 being determined. If the position of the reception antenna 1 is obtained, then the position of the GPS satellite 5 is already known so that the azimuth angle .theta. can be calculated by a directional cosine of a position vector between the antenna 1 to the GPS satellite 5.
The element for executing the position calculating process from the received radio waves in order to obtain the position of the antenna 1 is the GPS position computing unit 4 which receives the radio wave from the antenna 1. Further, the element for performing the aforementioned calculation of .phi. and the calculation of (.phi.+.theta.) on the basis of the position data from the GPS computing unit 4 and the data received from the antennas 1, 2 is the GPS azimuth computing unit 3.
As described above, the azimuth angle to the base line length L and accordingly, the azimuth angle of the navigation vehicle calculated at the GPS azimuth computing unit 3 is presented as (.theta.+.phi.), which is then output as a digital signal therefrom.
However, in the conventional azimuth angle measuring apparatus which makes effective use of the GPS satellite, the measuring process of azimuth angle takes plenty of calculation time and consequently the azimuth angle cannot be measured continuously. As a consequence, when a ship, for example, turns, an error occurs in the azimuth angle measuring process because of a delay of time.
Further, the GPS radio wave has an area and a time in which a measuring error is increased from a GPS satellite location standpoint. In addition, due to a magnetic abnormality caused by the activity of sun, the measuring process becomes difficult.
As a method for obviating the aforesaid shortcomings, an azimuth angle measuring method is proposed, in which an angular velocity sensor (e.g., rate gyro) and an azimuth angle measuring apparatus employing the aforementioned GPS are combined. However, according to the azimuth angle measuring method in which the above-mentioned angular velocity sensor and the GPS azimuth angle measuring apparatus are combined, when an angular velocity detection axis (hereinafter referred to as an input axis) of the angular velocity sensor is inclined during the ship turns, there is then the disadvantage such that an error occurs in the azimuth angle detected by the angular velocity sensor.
Further, in the GPS azimuth angle computing apparatus, there is then the disadvantage such that a signal is suddenly fluctuated considerably by the influence of a multipath of radio wave and a propagation state of radio wave.